The present invention relates to a field emission electron gun with an improved emitter and a method for fabricating the same.
A conventional field emission electron gun with molybdenum cone emitters which are sharp-pointed is disclosed in Journal of Applied Physics, Vol. 47, No. 12, December 1976. It is necessary to process molybdenum at high accuracy to form the molybdenum cone emitters on a silicon substrate. It is, in fact, difficult to process molybdenum at a high accuracy. For this reason, it is effective to use silicon for cone-shape emitters since it is relatively easy to process silicon at a high accuracy. In the Japanese laid-open patent applications Nos. 4-94033 and 6-52788, it is disclosed to use silicon for cone-shape emitters in the field emission electron gun.
In order to obtain a stable current property of the field emission electron gun, it is effective to connect a high resistance in series to the emitter such as a silicon base emitter. One of the typical conventional field emission electron gun is disclosed in the Japanese laid-open patent application No. 6-20592, a structure of which is illustrated in FIG. 1, wherein an illustration of a collector electrode is omitted. In practice, many field emission electron guns are provided in matrix on an n-doped silicon substrate 1. An emitter electrode, which is not illustrated, may be provided on the bottom of the n-doped silicon substrate 1.
An emitter, which has a cone-like shape and is sharp-pointed at the top, is selectively provided on the top of the n-doped silicon substrate 1. An emitter tip 9, which is made of a polysilicon highly doped with an n-type impurity, is formed at the head of the emitter. The base of the emitter is made of the same material as the silicon substrate 1. The emitter base has a higher resistivity than the resistivity of the emitter tip 9. An insulation film 5 is provided on the top of the silicon substrate 1, to encompass and to be spaced apart from the emitter. A gate electrode 6 is provided on the top of the insulation film 5, to encompass and to be spaced apart from the emitter tip 9.
Anther conventional field emission electron gun is disclosed in the Japanese laid-open patent application No. 5-36345, a structure of which is illustrated in FIG. 2, wherein an illustration of a collector electrode is omitted. In practice, many field emission electron guns are provided in matrix on an n-doped silicon substrate 1. An emitter electrode, which is not illustrated, may be provided on the bottom of the n-doped silicon substrate 1.
An emitter, which has a cone-like shape and is sharp-pointed at the top, is selectively provided on the top of the n-doped silicon substrate 1. The emitter comprises a head, which is made of a low resistive epitaxial silicon 11, and a base, which is made of a high resistive epitaxial silicon 10. The emitter base 10 has a higher resistivity than the emitter head 11. An insulation film 5 is provided on the top of the silicon substrate 1, to encompass and to be spaced apart from the emitter. A gate electrode 6 is provided on the top of the insulation film 5, to encompass and to be spaced apart from the emitter tip 9.
As described above, the head of the emitter has a lower resistivity than that of the base thereof, in order to reduce the ward function associated with the emitter and improve the discharge property. The high resistive base of the emitter can suppress a current fluctuation and obtain a stable discharge current.
As described above, in order to obtain a stable discharge current, it is effective to connect the high resistance in series to the head of the emitter. In designing the field emission electron gun, it is important to precisely control the resistance of the highly resistive portion connected in series to the head of the emitter. If the resistance of the emitter is increased, then the stable discharge current is obtained. It is necessary to design the emitter so that the resistance thereof is equal to or above a predetermined minimum value necessary for obtaining the stability of the discharge current. On the other hand, the high resistivity of the emitter causes a potential drop when a current flows through the emitter. It is necessary to raise the voltage to be applied to the gate electrode by an mount corresponding to the potential drop. The variation in the resistance of the emitter causes in the variation of the potential drop, thereby resulting in a variation of the gate electrode voltage. The resistive part of the emitter should be highly resistive and free from any variation in resistance.
In order to obtain a desirable resistivity, it is necessary that the impurity concentration is equal to or less than 1.times.10.sup.14 cm.sup.-3, when the resistive part of the emitter is made of an impurity doped silicon or an impurity doped epitaxial silicon. In this case, however, it is difficult to precisely control the resistivity of the impurity doped silicon or the impurity doped epitaxial silicon, thereby resulting in difficulty in controlling exactly the resistance of the emitter.
In place of the impurity doped silicon or the impurity doped epitaxial silicon, it is available to use a polysilicon doped with an impurity for the resistive part of the emitter. In this case, the resistivity depends on not only the impurity concentration but also grain size. The matured grain size depends on a temperature of the heat treatment for forming the polysilicon film. Actually, it is, however, difficult to control precisely the temperature of the heat treatment. For this reason, the grain size of the polysilicon film is likely to be variable and not uniform. As a result, the resistivity of the polysilicon film is likely to be variable. Thus, it is difficult to precisely control the resistance of the resistive part of the emitter.
In the above prior art, the head of the emitter is made of a material with a lower resistivity than that of the base of the emitter, in order to prevent any thermal destruction of the head of the emitter. Actually, it is unavoidable that an excess electrical current may accidentally and temporally flow through the emitter at over a predetermined maximum regulation value. The emitter structure is designed so that, even if such excess current at over the predetermined maximum regulation value flows through the emitter accidentally, then only the emitter head, with a low resistance, may be free from any heat destruction and melting. The emitter base is, however, made into the heat destruction or melting states due to its high resistivity, thereby causing a large destruction of the emitter, so that a short circuit may be formed between the emitter and the gate electrode. As a result, it is no longer possible to cause a potential difference between the gate electrode and the silicon substrate by applying a bias between them. This means that it is impossible to apply a gate voltage to the gate electrode. In practice, many field emission electron guns are provided in matrix on a silicon substrate. If the short circuit between the emitter and the gate electrode is formed in at least one of the field emission electron guns, then it is no longer possible to apply the gate voltage to the gate electrode of the remaining field emission electron guns, in which no short circuit between them is formed.
It has been required, for a long time, to develop a novel field emission electron gun with an improved emitter structure, which is free from the above problems.